1. Field of the Invention
This invention relates to an optical system of an optical information recording/reproducing apparatus in which a semiconductor laser is used as a light source, and further relates an objective optical system and a chromatic aberration correcting element suitable to the optical system.
2. Description of the Prior Art
An optical system of an optical information recording/reproducing apparatus such as optical disk apparatus, etc. comprises, as generally shown in FIG. 52, a light source portion 10 for emitting a generally parallel luminous flux, an objective optical system 20 for converging (focusing) the luminous flux emitted from the light source 10 onto an optical disk OD, a beam splitter 30 for splitting the luminous flux disposed between the light source 10 and the objective optical system 20 and adapted to split the luminous flux reflected by the disk, and a signal detecting optical system 40 for receiving such split luminous flux.
The light source portion 10 has a semiconductor laser 11, a collimator lens 12, and a beam shaping element 13.
The objective optical system 20 includes an objective lens 21, and a mirror 22, and disposed within a head 50 which is slided in the radial direction of the optical disk. Also, the objective lens 21 is mounted on an actuator (not shown) disposed within the head 50 and designed such that the objective lens 21 can be finely moved at least in the optical axis direction thereof so that a out of focus caused by warping of the disk, etc. can be corrected.
The signal detecting optical system 40 has a beam splitter 41, a tracking signal detecting system 42, and a focusing signal detecting system 43, and is adapted to reads information recorded in the disk and error signals of the trucks by reflected light from an optical disk OD.
By the way, an emitting light wavelength of the semiconductor laser used as a light source is shifted by changing of output power power and/or temperature. Because of the foregoing reason, when the chromatic aberration of the optical system is not corrected yet, the position of a light converging point is varied by shifting the wavelength.
When the light converging position is not coincident with the recording surface of the disk, there is a high possibility that incorrect writing and reading are effected.
However, out of focus due to comparatively gentle change of a wavelength caused by change of temperature or the like is automatically corrected by the afore-mentioned focusing servo when the collimator lens is corrected in chromatic aberration and change of temperature.
However, at the time when a data is written, an oscillation wavelength of a semiconductor laser is instantaneously shifted by several nm between a region where the temperature is increased and a region where the temperature is not increased. And the out of focus caused by such radical shift cannot be corrected by the above-mentioned focusing servo.
Therefore, especially when writing is effected, correction of the chromatic aberration of the objective optical system is important.
An optical system in which the objective lens itself is corrected in chromatic aberration is disclosed in, for example, Japanese Patent Early Laid-open Publication No. Sho 63-10118, Japanese Patent Early Laid-open Publication No. Sho 60-232519 and Japanese Patent Early Laid-open Publication No. Sho 58-72114.
However, the lens of the Japanese Patent Early Laid-open Publication No. Sho 63-10118 is of three pieces structure including an aspherical lens, while the lenses of the Japanese Patent Early Laid-open Publication No. Sho 60-232519 and the Japanese Patent Early Laid-open Publication No. Sho 58-72114 are of four pieces structure of glass lenses. Accordingly, there are such problems as that these lenses are heavy in weight compared with lenses which are not corrected in chromatic aberration, and a load incurred to a movable actuator is large.
As an objective lens for an optical disk apparatus is moved at a high frequency for the purposes of focusing and trucking, it is strongly demanded that the objective lens is made compact in size and light in weight in order to reduce the burden to the actuator.
Also, Japanese Patent Early Laid-open Publication No. Sho 62-269922 discloses an optical system for correcting the chromatic aberration of an objective lens by excessively correcting the chromatic aberration of a collimator lens. With this construction, it is necessary to excessively correct even a focusing error detecting optical system because otherwise out of focus is generated caused by a focusing servo.
However, the correcting amount of the chromatic aberration of the focusing error detecting optical system is proportional to a second raised power of the ratio M between a focal length of a condenser lens of this optical system and a focal length of the objective lens. Therefore, in an ordinary optical disk apparatus taking a value of about M=10 in view of the size of a light receiving element, it is difficult to design as such that the condenser lens has a sufficient correcting amount of chromatic aberration.